Afarensis

The rest of posterior basicranial growth in nonhuman primates occurs through posterior drift of the foramen magnum, which has been shown by fluorochrome dye labeling experiments to migrate caudally in nonhuman primates through resorption at its posterior end and deposition at its anterior end (Michejda, 1971; Giles et al., 1981). In contrast, the foramen magnum remains in the center of the human skull base, roughly halfway between the most anterior and posterior points of the skull (Lugoba and Wood, 1990).

The Primate Cranial Base: Ontogeny, Function, and Integration
Lieberman, Ross, and Ravosa
YEARBOOK OF PHYSICAL ANTHROPOLOGY 43:117-169 (2000)

The posterior drift of the foramen magnum in nonhuman primates is controlled by Hox genes - or at least that is what we infer based on studies in mice. There are a few studies, but not many, that seek to integrate recent developments in evo-devo into paleoanthropology. There are none that seek to integrate modularity theory into paleoanthropology

So I was quite happy to receive a copy of The Upright Ape: A New Origin of the Species by Aaron G. Filler. The Upright Ape is a well written, sometimes polemical, attempt to combine recent advances in genetics, evo-devo and paleoanthropology into a theory about bipedality. Although the aim of the book is to explain the origins of bipedality, it actually ranges over a wide variety of fields. The first couple of chapters are devoted to the vertebral theory suggested by Goethe, the adventures of Etienne Geoffroy Saint-Hilaire in Egypt (and the impact of Egyptian iconography on his thought), and Geoffroy Saint-Hilaire's idea about the inversion of the dorsal-ventral axis in vertebrates. The next three chapters discuss chromosomal speciation, taxonomy and homology, and modular selection theory. These chapters form the foundation for the final four chapters where Filler attempts to explain five major events in the history of life. These five events are the Cambrian Explosion, the origin of the vertebrates, the appearance of the dinosaurs, the mammalian diversification, and the origins of bipedality. For Filler all of these events share a common denominator. Namely a change in body plan that is ultimately related to Hox genes acting on higher order modules, such as the somites.

One example will have to suffice. Morotopithecus is a large bodied species of primate dating to the early Miocene (approximately 20.6 MYA). Morotopithecus has been called enigmatic because of its wide variety of traits, some of which link it to primitive catarrhines, others to modern hominoids. Cranially, em>Morotopithecus is characterized by a narrow interorbital region, broad nasal aperature, broad palate, pronounced canine jugum, long snout and short maxilla. Dentally it is characterized by procumbent incisors, large, sexually dimorphic canines, large quadrate molars (both primitive catarrhine traits), and broad premolars (such as in extant apes). Postcranially Morotopithecus has a rounded and superiorly expanded glenoid fossa (similar to a chimps), the articular surface on the head of the femur resembles that of monkeys (suggesting lesser ability to engage in abducted postures), a broad, shallow patellar grove, and thick diaphyseal cortical bone in the femur. The lumbar vertebrae share a number of features with modern hominoids such as a caudally inclined spinous process and a transverse process that arises from the pedicle. The lumbar vertebrae are also cranio-caudally long (unlike in modern hominoids). In most mammals, including hylobatids the lumbar transverse process is serially homologous with the rib. Most mammals, including lemurs, monkeys, hylobatids, and Proconsul africanus have a styloid process on their lumbar vertebrae. Humans, great apes and Morotopithecus don't. Filler argues that the reason for this is because the transverse process had switched and become serially homologous with the spinous process (well, with the structures giving rise to the spinous process). This indicates a fundamental body plan change - it moves, for example, the muscle attachments away from the midline increasing their leverage. It also allows for more upright posture. In the case of Morotopithecus more upright posture means fully bipedal. This has certain implications for the way apes and humans evolved (apes evolved away from bipedality - hence knuckle-walking is derived and bipedality the primitive condition) and Filler doesn't shy away from spelling these issues out.

For a book with only 260 pages Filler manages to pack an amazing amount of information into it, his discussion of the effects of rib and spine morphology among dinosaurs and mammals is a good example. At the same time the book doesn't suffer, in terms of readability, because of that. Although I can't say that I agree with everything in the book (leaving Morotopithecus aside, chromosomal speciation is on shaky ground, for example) I will echo Pilbeam, from the introduction, and say that this is a challenging and provocative book well worth reading.